Fibriform heat insulating materials have in the past included various organic heat insulating materials made of animal hairs, vegetable fibers, synthetic fibers, etc. and various inorganic heat insulating materials of glass fibers, asbestos, rock wool, ceramic fibers, carbon fibers, etc. Particularly, heat insulating materials for use at high temperatures have included inorganic heat insulating materials; however, they have not yet been fully satisfactory under certain application conditions.
For example, glass fibers begin to lose their strength at approximately 400.degree. C. and asbestos is dehydrated and becomes brittle at approximately 550.degree. C. And, as for rock wool and ceramics fibers, for instance, "KAOWOOL" (trade name), for example they are changed in their crystalline nature or are devitrified to become brittle, above about 800.degree. C., so that they can no longer maintain their heat insulating property when subjected to vibrations. Although the ceramic fibers of alumina, zirconia, or the like are used for the special application at superhigh temperatures, processes for their manufacturing are complicated so that it is difficult to make them in large quantities and they become very expensive products. And, as for carbon fibers, if exposed to high temperatures of approximately 400.degree. to 500.degree. C. or higher, they are oxidized and lose weight, so that the heat insulating property thereof cannot be maintained, and in addition, they are highly expensive.
Thus, none of these heat insulating materials is acceptable as one which is capable of maintaining a stable heat insulating property for a long time when applied to an outer shell, a flue, a steam pipe of an exhaust pipe, etc., of such arrangements which are to be subjected to high temperatures and vibrations as boilers, internal combustion engines, turbines, etc.